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Timescales of hydrothermal scavenging in the South Pacific Ocean from 234 Th, 230 Th, and 228 Th

Abstract : Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble uranium (234 Th, 230 Th) and radium (228 Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (234 Th) to years (228 Th) to tens of thousands of years (230 Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of 234 Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of 234 Th and 230 Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of 228 Th/ 228 Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total 234 Th activity close to that of its parent 238 U, but 230 Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the 230 Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.
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Frank Pavia, Robert Anderson, Erin Black, Lauren Kipp, Sebastian Vivancos, et al.. Timescales of hydrothermal scavenging in the South Pacific Ocean from 234 Th, 230 Th, and 228 Th. Earth and Planetary Science Letters, Elsevier, 2019, 506, pp.146 - 156. ⟨10.1016/j.epsl.2018.10.038⟩. ⟨hal-02511018⟩

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